1. Field of the Invention
The present invention relates to sound absorption technology for reducing noise, and more particularly relates to sound absorption technology that is effective in the suppression of low frequency turbo-machinery noise of a gas turbine or the like used in aviation, ships, and power generation, as a countermeasure against low frequency vibration and noise in large ducts for intake and exhaust at plants and the like, in noise reduction for air conditioning and household appliances, and for making a more comfortable work space at a construction site and the like, and a more comfortable traveling environment inside a vehicle, a passenger compartment, and the like.
2. Description of the Related Art
Well-known and conventionally used examples of general sound prevention means are: a sound insulation system that uses a sound insulating wall of the kind illustrated in FIG. 6-1, and sound absorption systems such as a resonance-type sound absorption system whose action is one of sound propagation interference based on impedance mismatching, or a resistance-type sound absorption system that induces a conversion into thermal energy produced by friction with a filler or loss at an opening. In an aircraft engine, a sound absorption device that has an ordinary honeycomb structure like that shown in FIG. 6-2, or a bulk-type sound absorption device, are principally employed. The former, the honeycomb type device, utilizes two-fold sound attenuation resulting from impedance mismatching by means of the resonance of a honeycomb chamber and from resistance when air passes through a throat portion. For a broader applicable frequency band, a sound absorption device has been put into practical use that has a dual-layer or multi-layer honeycomb structure in which honeycomb chambers of different capacity are interconnected through small openings. Generally, honeycomb-structure sound absorption devices are unsuitable for use in severe high-temperature environments. With the latter bulk type device, there is a correlation between the bulk filling factor and the frequency band, working from the fundamental principle of sound attenuation resulting from friction of incident sound wave with material in the bulk. In recent years, the application of ceramic-type heat-resisting material in high-temperature jet engines has been investigated. In order that these sound absorption devices should afford optimum noise reduction effects, honeycomb chamber capacity, degree of opening, filling factor, installation area, and the like, are selected as design points. Although the sound absorption devices mentioned above have a certain absorption performance, these devices also exhibit the problems of generating a pressure loss, a weight increase, and performance degradation beyond the scope of the design points.
As far as fan noise is concerned, the fan noise frequency falls in accordance with an increase in the bypass ratio. As a result, the honeycomb chamber capacity increases, which leads to an increase in engine weight and a pressure loss during cruising. On account of also not being able to track a change in noise frequency characteristics in accordance with a change in the fan rotation speed, there are limitations on the amount of noise reduction with conventional sound absorption devices, reduction amount. Also with regard to jet noise, there is a method that reduces noise in two stages, by first making a small scale vortex structure in a mixing acceleration device, such as a mixer, to shift the noise frequency band toward higher band, and then diminishing this higher frequency noise by sound absorbing material or by distance attenuation. However, this method is accompanied by the problem that thrust loss generated by the mixer components and a weight increase for the ejector are inevitable and that the provision of a demountable mechanism for the mixer causes a further weight increase. A general low noise problem, for example, with respect to low frequency sound, such as resonance generated within a long duct of large diameter is such that it is problematic to install existing sound absorbing material in a limited space. When a branch resonance pipe is used, in addition to there being an increase in the dimensions of the branch pipe, performance degradation beyond the scope of design points is considerable.
On the other hand, it has become apparent from the latest research that a phenomenon occurs according to which, when low-frequency sound waves impinge on a jet that passes through a hole and is ejected into a free air space, there is sound energy loss, and this phenomenon has been a matter of interest among researchers. In research to date, in predictions using theoretical models, simple modeling has been tested with regard to small-hole diameter, jet flow speed, degree of opening, back layer thickness, and the angle of incidence of sound waves with respect to a jet. However, there are few examples of sound absorption effects being examined, and, barely any verified results for cases of using complicated hole shapes, in particular, have been found. Further, as yet, there are no cases of practical uses being performed to optimize sound absorption efficiency by actively changing the aerodynamic and acoustic qualities surrounding the jet passing through a small hole.
It is an object of the present invention to provide, in the context of sound canceling technology of the kind described above, a sound canceling technology, which,
(1) absorbs low frequency sound including discrete frequency sounds and broadband frequency sounds to the same degree as existing sound-absorbing lining, and is capable of markedly reducing installation area in comparison with sound-absorbing lining;
(2) is capable of suppressing growth of an acoustic mode in an air space even without an adjustment mechanism by means of optimization of the shape of the small holes even in a high sound pressure and high temperature environment of a combustion chamber or similar;
(3) is capable of maintaining sound absorption efficiency by tracking a change in the noise characteristics of a low frequency sound source that has wavelengths equal to or greater than a small hole diameter, also in a case where, at a reference frequency, 50% of acoustic design points fail; and
(4) is capable of minimizing weight increase produced by movable devices that are visible in a sound-absorbing panel, the consumption of electrical energy for drive, pressure loss, and the like, and effects on operating performance of the main system.
The fine jet control-type sound absorption system of the present invention comprises: a barrier wall, which partially covers an air space in which noise propagates and which has a multiplicity of small holes formed therein; and means for causing a fine gas flow passing through these small holes to flow into or out from the airspace, which sound absorption system generates, by means of the passage of gas, a fine jet in the air space in the vicinity of the barrier wall, to cancel noise, wherein any of or a combination of the shape of the small holes, a jet ejection angle or entrance angle, a jet flow amount, the thickness of a barrier wall back layer, and an air flow angle at which air flows into the barrier wall back layer, is selected.
Further, in order to enhance sound absorption effects, the fine jet control-type sound absorption system of the present invention further comprises: an adjustment control mechanism, which installs an error sensor in a noise air space, and which, so that the output of this error sensor is minimized, regulates any of or a combination of the shape of the small holes, a jet ejection angle or entrance angle, a jet flow amount, the thickness of a barrier wall back layer, and an air flow angle at which air flows into the barrier wall back layer. For the shape of the small holes, an optional shape can be selected for the barrier wall opening cross-section and a minute protrusion may also be formed at the leading and trailing edges of the opening in accordance with requirements, and, in a barrier wall thickness direction, an opening formed with a multi-layer step, a taper, a hollow or a protrusion can be selected, it being possible to selectively employ any of or a combination of the above shapes and constitutions.
Further, the fine jet control-type sound absorption system of the present invention is provided with a plurality of barrier wall portions which have a different degree of opening, equivalent diameter, small-hole shape and jet speed, and the like, permitting optimum sound absorption effects for each frequency band, and which are employed in suitable combination in accordance with noise frequency, wherein an adjustment control mechanism has a constitution in which a high-performance material, such as a piezoelectric element, a shape memory alloy, and the like, is embedded in the vicinity of a small hole or in part of the barrier wall, and, by slightly changing the shape of a small hole in accordance with a control signal from outside, controls a separation phenomenon of shear eddies generated by the jet to adjust the sound absorption band and sound absorption amount.
Further, in the fine jet control-type sound absorption system of the present invention, barrier walls, which are formed having a multiplicity of small holes, are installed in multiple layers, and means for displacing individual barrier walls is provided, so that, by shifting a relative positional relationship between small holes, an improvement of sound absorption characteristics is made possible.